The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
With the advent of the space age in the mid-twentieth century, global satellite navigation systems (GNSS) developed to allow autonomous geo-spatial positioning, often with global coverage. Such global coverage is generally achieved by a constellation of satellites, often 20 or more, in medium Earth orbit. Carefully timed signals received from the satellites in known orbits allow the receiver of the information to determine, with varying degrees of accuracy, the distance of the receiver from each satellite. Using signals from four or more satellites (three if the altitude of the receiver is known), the position of the receiver can be determined with relative accuracy. The Global Positioning System (GPS), also known as Navstar, is one implementation of a GNSS, and is maintained by United States government. Depending on the receiver, atmospheric effects, errors in calculated satellite orbits, multipath distortion, and clock errors, among other things, GPS has an accuracy on the order of 1-10 meters.
In some situations, available GPS signals do not provide sufficient accuracy for the intended application. For example, while accuracy on the order of one meter (or even five meters) may be sufficient for GPS implemented in the automotive environment (e.g., to provide navigation information to automobile drivers), certain other applications, such as surveying and techtonics (e.g., direct measurement of fault motion), require much greater accuracy, on the order of a few millimeters in the case of surveying. In other situations, GPS signals are unavailable or subject to degradation. In tunnels, for example, GPS signals are often unavailable to the GPS receiver, rendering the GPS device unable to calculate any position at all. In other environments, such as city streets having tall buildings on either side (known colloquially as “urban canyons”), the buildings may block one or more GPS signals entirely, and multipath effects may degrade receiver accuracy. A number of technologies exist to increase the accuracy of the GPS system, including, among others, Assisted GPS (AGPS), Differential GPS (DGPS), GPS with dead reckoning, and map matching.
Assisted GPS (AGPS) can improve the performance of a GPS satellite-based positioning system. AGPS achieves performance gains by, among other things, using data available from a network, such as a mobile (e.g., cellular) telephone network or the Internet, to: (1) assist GPS receiver devices in attaining initial GPS signal acquisition by, for example, supplying orbital data of the GPS satellites to enable the receiver to lock to the satellites more quickly; (2) provide knowledge of conditions (e.g., ionospheric conditions) affecting local GPS signals, thereby decreasing the errors affecting accuracy; and/or (3) utilizing one or more servers to calculate the position of the GPS receiver device, to reduce the load on or delays associated with the processor in the GPS receiver.
Differential GPS (DGPS) uses data about the difference between positions indicated by the satellite systems and known fixed positions. The data are generated and broadcast by a network of fixed, ground-based reference stations. Different geographical regions operate different DGPS systems, with no single standard dominating. Europe, Canada, and the United States each have different DGPS systems operating within their borders.
In some GPS systems, dead reckoning (DR) and/or inertial navigation (IN) allows estimation of position during periods where GPS signals are degraded or unavailable. Speed sensors (e.g., the speedometer on an automobile), radar systems, electronic compasses, accelerometers, and the like may provide data to the GPS device to use with the data last calculated using GPS signals. Throughout this specification, systems that implement a combination of GPS and DR and/or IN are referred to generally as GPS+DR systems.
In still other systems, a GPS device calculates a position based on received GPS signals and, by referring to one or more maps of the area around the calculated position, corrects the position to match a probable location. For example, in an automotive GPS device, the device may assume that the vehicle in which it operates is on a road and, accordingly, may determine that the position of the device is at a point on a road closest to the position calculated from received GPS signals. Throughout the specification, the method implemented by such systems is referred to as “map matching.”
AGPS, DGPS, GPS+DR, and map matching, while improving position accuracy and/or availability, all have significant drawbacks. AGPS and DGPS, for example, each require additional networks of transmitters and/or receivers, as well as additional receivers (and sometimes transmitters) in the GPS device. GPS+DR systems require additional sensors, located within or at least in communication with the device and, because new positions are calculated from old positions, the position fix accumulates over time. Map matching requires often significant digital storage space to store maps, especially to cover large geographical areas or dense urban areas. Map matching also suffers degradation of accuracy as maps become outdated.